Replace custom A2A client/server implementations with official A2A SDKs

[PetrBulanek]

Is your feature request related to a problem? Please describe.
Both the UI and server use hand-rolled A2A v1 implementations:

• TS (adk-ui): custom JSON-RPC/SSE client, agent card resolution, streaming logic, message/task lifecycle helpers
• Python (adk-server): custom A2A proxy routes and service layer

These work, but duplicate what the official SDKs should provide and risk drifting from the spec.

Describe the solution you'd like
Once the official SDKs are stable, replace both custom implementations:

• TS client → official A2A TypeScript SDK
• Python server → official A2A Python SDK (currently pinned to 1.0-dev)

Describe alternatives you've considered
Keep the custom implementations long-term — rejected due to maintenance burden and spec drift risk.

Additional context
Blocked on SDK stabilization. Tracking upstream:

• TS: https://github.com/a2aproject/a2a-js
• Python: https://github.com/a2aproject/a2a-python

[PetrBulanek]

Blocked on upstream: this requires @a2a-js/sdk with v1 spec support, which is tracked in a2aproject/a2a-js#321 (currently 0/13 subtasks complete).